1. Field of the Invention
This invention relates to cameras, and particularly to an automatic focusing device which automatically shifts its focusing accuracy according to the exposure control mode selected for the camera.
2. Description of the Prior Art
Generally, when a camera's depth of field is made shallow by a wide open aperture, the resulting pictures tend to be cut-of-focus unless the focusing accuracy is very high. Conversely, a long depth of field produced by a stopped-down aperture may result in a sharply focused picture even when the main object to be photographed is located away from an in-focus position as long as the object is within the limits of the depth of field. It is conceivable to lower the focusing accuracy when the depth of field is greater than a certain given value and increase the accuracy when the depth of field is shallower than the given value. This concept has prompted development of a method of shifting the degree of focusing accuracy on the basis of an aperture value obtained either from an photometric computation circuit or an aperture ring. This method is disclosed, for example, in U.S. patent application Ser. No. 59,635, U.S. Pat. No. 4,157,217, etc.
However, photographers are sometimed forced by an insufficient light condition, such as the dusk of the evening or a cloudy sky, to photograph with the lens aperture open as wide as possible. This results in a shallow depth of field. If, in that event, the shallow depth of field causes the camera to detect the focus automatically with greater degree of focus detection accuracy, the focus detecting operation may take too long to seize an unexpected picture-taking opportunity. Therefore, determining the focusing accuracy merely on the basis of the photo-taking aperture value hardly makes it possible to carry out focus detection operations at different degrees of accuracy suited for different photographing modes such as a panning mode which requires quick shooting, a portrait mode, etc.